Passive cooler

ABSTRACT

A passive cooler arranged for three stages of cooling including an outer L-shaped member serving as two of the cooling stages and an inner member serving as the third stage of cooling at a preselected equilibrium temperature. Certain surfaces of each leg of the outer member and an exposed surface of the inner member are each given a surface finish having a spectral response in accordance with certain radiation wavelengths such that the inner member is maintained at a preselected temperature which is lower than otherwise possible when the cooler is exposed to thermal radiation, the cooler being located in an environment having a temperature lower than the preselected temperature.

United States Patent [191 Williams [11] 3,817,320 June 18, 1974 PASSIVECOOLER 75 Inventor: Richard Jean Williams, Marlton,

[73] Assignee: RCA Corporation, New York, NY.

[22] Filed: 7 Mar. 2, 1971 [21] App]. No.: 120,076

[52] US. Cl 165/47, 165/80, 165/133 [5l] Int. Cl. F24h 3/00 [58] Fieldof Search 165/47, 80, 133, 44

g [56] References Cited UNITEDSTATES PATENTS 3,550,678 l2/l970Pfouts....= 165/44 Primary Examiner-Charles Sukato vAttorney, Agent, orFirm Edward J. Norton; William;

Squire [57] ABSIRACT A passive cooler arranged for three stages ofcooling including an outer L-shaped member serving as two of the coolingstages and an inner member serving as the third stage of cooling at apreselected equilibrium temperature. Certain surfaces of each leg of theouter member and an exposed surface of the inner member are each given asurface finish having a spectral response in accordance with certainradiation wavelengths such that the inner member is maintained at apreselected temperature which is lower than otherwise possible when thecooler is exposed to thermal radiation, the cooler being located in anenvironment having a temperature lower than the preselected temperature.

10 Claims, 3 Drawing Figures SPACECRAFT 20 K SPIN AXIS so COOLER QPAI'ENTEDJun 18 m4 sum 1 0F 2 \WSUN 50 ISPACECRAFT 20 IM'ORBT PLANE 40SUN ANGLE B INVENTOR. Richard J Williams BY ATTORNEY P'ATENTEDM 18 m4SHEEI. 2 BF 2 I INVENTOR. v

Richard J Williams ATTORNEY PASSIVE COOLER BACKGROUND OF THE INVENTIONThis invention relates to passive coolers which are capable of cooling asupported article to a pre-selected temperature by radiation to thesurrounding environment in the presence of wide band thermal radiation,which, unless otherwise provided for, would heat the article above thepre-selected temperature.

Coolers have widespread application on spacecrafts for cooling sensitiveradiation detectors which are operative at cryogenic temperatures, thedetectors being used to scan the earth from an orbit position about theearth. One application for such detectors is in the weather satellitesnow in use. One such device which may be used with the cooler of thepresent invention is described in my copending application entitledAnti- Condensation Device for Infra-red Detector, Ser. No. 2

101,328 filed Dec. 24, I970, and assigned to the assignee of the presentinvention.

Several methods are available for cooling these detectors, the methodsincluding electro-mechanical refrigerant systems, combination of thermalelectric and radiation systems, solid cryogenic space coolers whichutilize solid cryogenic material which sublimes at a controlled rate,generating cooled gas for cooling the detector, and passive, radiationcoolers. The passive cooler is preferred since it has no defined lifelimitation, requires no external power, and has no moving parts. Theseoperating features lend themselves to make passive coolers smaller,lighter and relatively inexpensive in comparison with the other coolingmethods.

Nevertheless, a problem with passive coolers is overheating due toinability to dissipate thermal energy received from thermal sources suchas the earth, sun and spacecraft. More specifically, a passive coolerattached to a spacecraft orbiting the earth is subject to thermalradiation of the sun, the earth, and the spacecraft, jointlycontributing to heating a radiation detector above its operatingtemperature. In order to maintain thermal equilibrium in a body at adesired temperature, it is essential that the incident energy bebalanced with the radiated energy from the body for that temperature.Prior art coolers operate at equilibrium. temperatures that areunsatisfactory for most present-day passive cooler applications. Coolersshould provide an equilibrium temperature at; a preselected temperaturein the cryogenic region of radiation detectors in the order of 70K to100K.

Prior art passive coolers have been constructed which provide stagedshielding from thermal sources such as the earth, sun and the spacecraftand black body (entire thermal energy spectrum), radiation surfacesfacing that portion of space that serves asa thermal sink. In practice,such coolers cannot maintain. a preselected low temperature due torestricted thermal coupling to dark space. These coolers generally havebeen formed as hollow pyramids or cones of revolution in which thedetector is located in the interior of the apex and the black bodyradiating base is exposed to dark space. Such cooling devices are notsuitable to maintain temperatures in the ranges needed for present dayneeds.

SUMMARY OF THE INVENTION second wavelength range which tends to raisethe temperature of the body above the equilibrium temperature. Tomaintain the equilibrium temperature, the body is provided with asurface having high reflectivity and low absorptivity of the secondwavelength range and low reflectivity and high absorptivity at the firstwavelength range. By this arrangement the body is coupled to theenvironment with respect to radiant energy in the first wavelength rangeand decoupled to the environment with respect to radiant energy in thesecond 0 wavelength range, providing significantly improved thermalcoupling to the external environment to thereby achieve lower thermalequilibrium than has heretofore been achieved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammaticalillustration of the orientation of the cooler of the present inventionin space;

FIG. 2 is a fragmented perspective view of a cooler in accordance withthe present invention; and

FIG. 3 is a sectional view of the cooler of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT The cooler of the presentinvention is adapted to be located in an environment which is estimatedto have a temperature of 4K and an absorptivity factor of I. Space canbe considered to be a black thermal sink of infinite capacity andtherefore is an environment of use of the cooler of the invention.Further, space is assumed in the art to be a vacuum. Thus bodiesspatially separated with respect to each other in space are thermallyconductively and convectively insulated from one another. Accordingly,the only other means of known heat transfer in a space environment is byradiation. A body in such an environment if exposed to radiated thermalenergy from external sources such as the earth and sun will reach anequilibrium temperature higher than the temperature of space. Thetemperature of equilibrium that is established will depend upon thethermal absorption and radiation equilibrium of that body.

The spacecraft being exposed to radiation from the earth and the sun,has its temperature elevated above the 4K temperature of space, andtherefore, is also a radiator of thermal energy relative to certainportions of itself including a cooler which may be attached to it. Inpractice the radiation from the other celestial bodies including themoon and stars is negligible. Without further provisions, a body such asa cooler attached to an orbiting spacecraft would reach a temperatureconsiderably higher, for example, 200K or higher, than the cryogenicoperating range of a radiation detector, for example -80K.

To provide cooling within the cryogenic temperature range of radiationdetectors, the present invention provides for a cooler construction thatutilizes radiation, radiation shielding, and staging in which each stageis maintained at a progressively lower temperature by means includingconductive and radiation thermal isolation of the various stages fromeach other, noting that no thermal convection occurs in the vacuum ofspace. In accordance with the invention a selected portion of the cooleris maintained at a preselected cryogenic temperature. For example, aradiation detector in thermal contact with that cooled portion effectsconductive cooling of the detector at that preselected temperature.

Instead of using a black body type of surface, the cooler of the presentinvention is provided with several surfaces each of which differ intheir thermal energy response characteristics. In particular, thesurfaces are arranged to respond essentially to solar and infraredwavelengths. Solar thermal energy radiation generally includes energyradiated within the entire thermal radiation spectrum. However, most ofthe thermal energy emitted by the sun falls within the visible region ascompared to the portion of the radiation emitted in the infrared region.Thus the essential portion of solar radiation is in a differentwavelength region than the infrared radiation region for purposes ofpassive cooling according to the present invention. To utilize thesedifferent wavelengths means are provided which selectively eithercouples or decouples certain surfaces in each of the stages to radiationin the environment within pre scribed wavelengths. In particular, thesewavelengths may be characterized generally as infrared and as solar orvisible radiation. Both the radiation of thermal energy to dark space aswell as the radiation shielding effected in the cooler is a function ofnot only the orientation of the various radiating bodies such as theearth, sun and spacecraft with respect to the cooler but also of thewavelength of radiation from these bodies.

A preferred embodiment of the invention will now be described withreference to the drawing. In FIG. 1, cooler 10, to be described infurther detail in conjunction with FIGS. 2 and 3, is secured tospacecraft 20 by suitable mounting devices (not shown). Spacecraft 20orbits about the earth 30 in orbit plane 40. Spacecraft 20 spins aboutspin axis 60 once per orbit about earth such that cooler is alway in thesame position with respect to earth 30. Sun rays 50 are disposed atangle B to spin axis 60, angle [3 remaining constant in a manner to bedescribed.

In describing the preferred embodiment, with respect to FIGS. 1, 2 and3, like numerals refer to like or identical parts. Member 11 is formedof generally planar leg portions 13 and 14 and an interconnectingsemicylindrical portion 15. Member 12 is located in the cavity formed byportion as shown in FIG. 3. Leg portions 13 and 14 are located in spacedrelationship preferably at right angles with respect to each other,member 12 being located generally in the space between the two legportions 13 and 14. Legs 13 and 14 may be spaced at an angle greaterthan but not less than 90. Member 11 is further provided with two webshield portions 21 and 22. Web portions 21 and 22 are generallytriangular and planar in shape, and, together with leg 14 andinterconnecting portion 15, form a scooplike cavity.

The structure is arranged for the convenience of description into threeportions, each portion serving as one of three stages of thermallyindependent structures. Each stage is thermally conductive, isisothermal, and is at a different temperature with respect to the otherstages. According to the invention, the stages cooperate, in sequence,to reduce to a preselected tem perature the temperature of the article(radiation detector, for example) located in thermal contact with thethird stage. This pre-selected temperature is established and maintainedat that temperature inherently and without any control means usuallyconsidered necessary for temperature control.

Leg portions 13 and 14 and interconnecting portion 15 are preferablyconstructed such that the legs and interconnecting portion include anouter skin or housing 16. Housing 16 serves as a first stage radiationshield for the portions of the cooler nested therein to be described.Housing 16 is a thin sheet of metal which is structurally rigid andthermally conductive and is preferably a magnesium casting. Housing 16forms the outer sides of legs 13 and 14, interconnecting portion 15 andwebs 21 and 22. Edges 41-46, inclusive, of housing 16, are bent inwardas shown to form a shallow cavity. The outer side of web 21 is acoplanar extension of edges 41 and 43 while the outer side of web 22 isa coplanar extension of edges 42 and 44. Nested within the shallowenclosure cavity formed by housing 16 and adjacent to housing 16including webs 21 and 22 is a suitable multilayer thermal insulationblanket 25 formed for example of vapor deposited gold on H- Film,trademark of DuPont Company for a form of polyimide film, separated byalternating layers of nylon net. Insulation blanket 25 serves tothermally insulate the inner portions of the cooler from housing 16.

Disposed adjacent blanket 25 in the cavity formed by housing 16 areinner panels 17 and 18, frame 19, and inner web portions 47 and 48 ofwebs 22 and 21, respectively, which together serve as a second stage.Panel 17 is nested in leg 13 and panel 18 is nested in leg 14 whileframe 19 is nested in the concave cavity of portion 15. Frame 19 servesto mechanically and thermally interconnect panels 17 and 18. Frame 19 ispreferably formed of a magnesium casting while panels 17 and 18 arepreferably formed of aluminum honeycomb sheets. Frame 19, portions 47and 48 together with panels 17 and 18 are an integral rigid structure.The inner portions 47 and 48 of webs 21 and 22, re spectively and frame19 are preferably a magnesium casting. The second stage structure(comprising frame 19, panels 17 and 18, and inner web portions 47 and48) is secured in the nested position spaced from the first stage(housing 16) by thermal isolation mounts (not shown). The spacedrelationship between the first and second stage is defined by gap 39.Due to the vacuum of space, gap 39 permits the thennal conductiveisolation of housing 16 from the first stage. Blanket 25 providesradiation isolation between the first and second stages.

A socalled second surface mirror 38 having a surface 27 of a glass panel31 and a silver coating 32 is secured in thermal contact with panel 17.Any suitably bonding medium may be used to secure mirror 38 to panel 17.Silver coating 32 of mirror 38 is adjacent to and in thermal contactwith panel 17. Coating 32 is vapor deposited on glass panel 31. Panel 17serves to provide a rigid planar structural support for mirror 38 whichis preferably 6-8 mils thick, the silver being 5 microns thick. Bysecond surface mirror is meant in this art a mirror in which atransparent medium such as glass (31) is coated on one side with areflective medium such as silver (32). The reflective side of the coat-Leg portion 13 thus includes mirror 38, panel 17 and that portion of'blanket 25 and housing 16 within the bracket while leg portion 14includes panel 18 and that portion of blanket 25 and housing 16 withinthe bracket as illustrated in FIG. 3.

Member 12 serves as the third stage of cooler and includes a thin metalplate 29 and coating 28 having a surface 33. Member 12 is suspended fromframe 19 in interconnecting portion in thermal isolation therefrom bymeans not shown. An example of the thermal isolation means is describedin copending application entitled Suspension System, Ser. No. 79,496,filed Oct. 9, 1970 invented by David Melrose and Derek Binge andassigned to the assignee of the present invention.

The inner surfaces of cooler 10 consist of surface 26 of leg 14, surface33 of member 12, surface 27 of leg 13 and the inner surfaces,respectively, of web portions 47 and 48 each of which face each other.Each of these surfaces are adapted to have spectral properties whichcompliment one another, in a manner to be described, to provide improvedoverall thermal coupling to dark space (i.e. to the ambient external thespacecraft).

It should be understoodthat the spectral properties of a surface dependsin part on the subsurface portions, thus surface 27 includes both glasspanel 31 and silver coating 32 which together contribute to the spectralproperties of surface 27.

A brief description of the relationships of the various propertiesrelating to this invention will it is believed aid towards a betterunderstanding of the nature of the invention. The spectral properties ofmaterials can be shown to be related by the formulae:

where A, in general, is absorptivity throughout the entire frequencyspectrum of radiation, E is emissivity, 'r is transmissivity, and p isreflectivity of radiant thermal energy. For radiation receiving andemitting bodies at the same temperature, emissivity (E) of a body equalsabsorptivity (A) of that body. Conversely, for radiation receiving andemitting bodies at different temperatures, their respective propertiesof absorptivity and emissivity are not equal. Equation (1), when usedwith reference to solar energy usually includes the parameter a of theabsorptivity of a surface with respect to solar energy instead of themore general parameter A.

Emissivity (E) is the percent energy radiated by a body relative to thatenergy which would be emitted by a black body at the same temperatureand is a direct function of temperature of the emitting body.Absorptivity (a) of a body is, on the other hand, related to the energyimpinging upon the body, and' is a function of the temperature of theother body from which the energy is radiated. Thus, while a particularsurface may have low emissivity (E) of its own energy, it may also 6 beprovided with a relatively high absorptivity (a) of energy received froma higher temperature body, where the emitting body (sun) temperature ishigher than the receiving body (cooler) temperature.

In accordance with the present invention, passive cooling at lowertemperatures than heretofore obtained is effected by utilizing theprinciple that thermal radiation occurs over a wide band of wavelenths,and by providing certain surfaces of the cooler to be selectivelyresponsive to thermal radiation wavelengths within predetermined rangesof the wide band.

Thermal radiation is restricted for this embodiment into two wavelengthranges, essentially within the infrared region and the solar region eachof which having been defined above. Particular surfaces of the coolerare each provided with certain properties that have significantlydifferent responses to thermal radiation in accordance with thewavelength of the thermal radiation arranged to impinge upon thatsurface. These certain properties when arranged according to thisinvention provide improved thermal radiation coupling to dark space.These properties are arranged to cooperate to provide essentiallythermal coupling to dark space in the infrared region and thermaldecoupling in the solar (visible) region.

The preselected temperature of member 12 is achieved in accordance withthe present invention by providing the inner surface 26 of leg 14 with alayer of gold which is both highly specular, i.e. light reflectedtherefrom is columnar rather than diffuse, and highly reflective, i.e.energy that is not absorbed, with respect to bodies emitting energysubstantially essentially within the infrared region. High reflectivity,as used herein shall be understood to mean reflectivity (p) in the ordersubstantially of 0.95 to 1.00 while low emissivity (E) shall beunderstood to be in the order substantially of 0.05 to zero for theoperating temperature of surface 26. These properties are achieved forexample in a surface (26) of gold just described. At the expected lowoperating temperature of surface 26, the p and E thereof relate toradiation in the infrared region. Therefore, since surface 26 has a lowE, very little thermal energy is radiated therefrom. With respect tothermal radiation in the solar region, the gold surface 26 hasrelatively high absorptivity a. As indicated above, due to the muchhigher temperature of solar radiation as compared to the temperature ofsurface 26, emissivity E of surface 26 is low while its absorptivity ais high with respect to E, such that a/E is in the order of 10. Thus, itshould be appreciated that while surface 26 absorbs little radiationwithin the infrared region, surface 26 absorbs by a factor of 10radiation within the solar region.

Inner surface 27 of leg 13 is arranged by the structural arrangement ofthe second surface mirror 38 to have a low solar absorptivity a in theorder of 0.1 and high infrared emissivity E in the order of 0.85. Aspreviously described, it is to be noted that second surface mirror 38 isthermally coupled to panel 17.

Glass panel 31 has high emissivity in the order of 0.85 and therefore,high absorptivity within the infrared region while silver 32 has highreflectivity within the solar region. Further, the silver coating isspecular, that is, there is no scattering of light impinging thereon.Thus, earth reflected solar energy impinging upon surface 27 of mirror38 is reflected into space predominantly by reflections from the silvercoating 32 while earth emitted infrared energy is absorbed by the glasspanel 31. The amount of absorbed infrared energy is a function of thethickness of glass panel 31 which is chosen according to designrequirements.

Surface 33 of member 12 is the exposed surface of coating 28 and isarranged to be highly emissive in the infrared region suitably in theorder of 0.9. Either black or white paint provides such high emissivity.According to the embodiment, coating 28 is conventional space stable,white paint, i.e., white velvet-400 series manufactured by the 3MCompany, providing high reflectivity in the solar region in the order of0.8. The inner surfaces of web portions 47 and 48 which face each otherare coated with vapor deposited gold, suitably polished to highspecularity in a manner known in the art to provide high reflectivity(approximately 0.95) in the infrared region and relatively high a in thesolar region, a/E being in the order of similar to surface 26 aspreviously described.

All interior surfaces of housing 16 and panels 17 and 18, frame 19, webs21 and 22, and member 12 are also suitably vapor deposited with gold tominimize radiation coupling between the first and second stages andbetween the second and third stages.

The outer exposed surfaces of housing 16 are coated with conventionalwhite paint (not shown) which serves as a solar reflector and radiator.Alternatively, the outer surfaces of leg 13, webs 21 and 22 and sectionmay be covered with a suitable insulation blanket (not shown) while theouter surface of leg 14 may be covered or placed in thermal contact witha second surface mirror (not shown) in a similar manner as the innersurface of leg 13. This latter (alternative) covering of the outersurfaces of housing 16 improves the efficiency of cooler 10 in a mannerto be described.

In operation, cooler 10 is oriented as shown in FIG. 1. In thisorientation, spacecraft orbits about earth 30 in plane 40 in a sunsynchronous, earth oriented orbit. By sun synchronous is meant that thesun angle ,3, the angle of the sun's rays 50 to the spin axis 60 ofspacecraft 20, is substantially constant, and is preferably a maximum of75 to prevent direct radiation by the sun on surface 26. By earthoriented is meant the spacecraft rotates at one orbit per revolutionabout the earth with cooler 10 always positioned in the same orientationwith respect to the earth.

In the position illustrated in FIG. 1, cooler 10 is secured at thebottom of the spacecraft adjacent the end 20a of the spacecraft oppositethe sun rays 50 such that the cooler is in the spacecraft shadow 70 withthe outer surface of leg 14 facing the earth. When the spacecraft ispositioned 180 from the illustrated position such that the spacecraft isat the bottom of the Figure, it is apparent that all of the outersurfaces of member 11 are exposed to sun rays 50, leg 14 still facingthe earth. Note also that the orbit plane 40 positions leg 14 to faceboth the light and the dark sides of the earth. Thus, the outer surfacesof member 11 are exposed to radiation sources of discretely differentbands of wavelengths, which nevertheless, do not substantially affectthe temperature of final stage member 12, which is always shielded fromdirect radiation from the sun, earth and spacecraft. in thisorientation, the spacecraft appears to cooler 10 as a body having atemperature of about 293K while the earth appears as a body having atemperature of 233K.

The inner surface of leg 13 and the exposed surface of member 12 arepositioned to be beyond the field of view of spacecraft 20. Thus, cooler10 is positioned at the extreme end 20a of spacecraft 20 opposite thesun. The inner surface of leg 13 is either coplanar with end 20a orlaterally displaced in the direction of spin axis 60 further from thesun than end 20a. Leg 14 extends generally in the horizontal directionwith inner surface 26 facing spacecraft 20 while leg 13 extendsgenerally in the vertical direction. The scale of the drawing is out ofproportion for purposes of illustration, but in practice, the scale issuch that the inner surface of leg 13 is exposed to dark space and earthradiation, while member 12 is always shielded from earths radiation byleg 14 and webs 21 and 22.

In the orientation of the spacecraft as described above, the exteriorsurfaces of cooler 10 are exposed to solar radiation. With an exteriorcoating of white paint (not shown) on housing 16, there is someabsorption of solar energy while most of the solar energy is reflected.Since the cooler is exposed to the sun only during a portion of theorbit, there is a change of temperature in housing 16. A portion of theenergy absorbed by housing 16 when exposed to the sun is later radiatedto space when the cooler is not exposed to the sun, resulting inexpected temperature variations of housing 16 within the range of 220 to320K.

By thermally isolating the second stage from the first stage, the secondstage is maintained at a lower temperature expected to be within therange of l40l60K. Since the first stage is thermally isolated from thesecond stage, the significant thermal input to the second stage isradiation from the earth received by mirror 38 on leg 13. However, sincemirror 38 is a reflector of solar energy and a radiator of infraredenergy, the equilibrium temperature in the range of l40-l60K is reachedwhich is significantly lower than the temperature of the first stage.

In the orientation shown, surface 26 is exposed to the spacecraft 20,but due to the highly specular finish of surface 26, the infrared energyradiated from the spacecaraft is substantially totally reflected bysurface 26 into space and not to leg 13 or member 12 since there issubstantially no scattering of radiation. Similarly, surface 26 isexposed to mirror 38 but, any radiation from mirror 38 is in theinfrared region and is also reflected by surface 26 out to space. It isapparent that there is substantially no absorbed radiation on the secondstage other than the thermal radiation from earth on mirror 38. Further,any thrrnal radiation absorbed by panels 17 and 18, and frame 19, byheat leaks due to radiation coupling with the first stage or housing 16is radiated to space by mirror 38. The undesired thermal coupling(generally, heat leakage) of member 12 (third stage) to the second stageis significantly reduced by minimizing radiation thermal couplingtherebetween using insulation such as blanket 25 and gold coating. Thecoupling can be further reduced by minimizing the conduction thermalcoupling therebetween by way of a thermal isolation support (not shown)using, for example, the support described in the copending applicationSer. No. 79,496, noted above between member 12 and frame 19. By coatingmember 12 with emissive (white paint) coating 28, the temperature ofthis third stage (member 12) can be even further reduced by radiation todark space. Coating 28 is advantageously made white to prevent bursts ofsolar radiation during the spacecraft launch from damaging the detectorconnected to the third stage. Leg 14 together with webs 21 and 22 shieldmember 12 from earth radiation.

It should be noted that the net heat flow between the stages varies withthe fourth power of absolute temperature, i.e., with T, where T is theabsolute temperature of a stage. Thus, as the temperature increases, theheat flow between stages increases at a much greater rate. It can beshown that where the housing 16 reaches a temperature above 300K, theheat flow between the first stage and the second stage is significantenough to cause some change in the second stage temperature owing to thehigher housing temperatures. However, due to the thermal attenuationbetween stages, only minor fluctuations occur in the third stagetemperature. For example, with the first stage varying between 220 to320K the second stage may vary between l40-160K and the third stage willvary only within a few degrees.

In some cases, it is desirable to provide even more efficient coolingand hold the third stage temperature to within a degree. In this latterinstance, the white paint coating on housing 16 is substituted by aninsulation blanket such as blanket 25 (previously described) over theouter surfaces of leg 13, webs 21 and 22, and section and a secondsurface mirror on the outer surface of leg 14 in thermal contact withhousing 16. Such an insulation blanket placed on the outer surface ofleg 13 and webs 21 and 22 will absorb thermal energy when the blanket isexposed to direct radiation by the sun, and, by conduction andradiation, this absorbed thermal radiation is transferred to housing 16.During a portion of the orbit when the sun is shielded from the outersurface of leg 13 by spacecraft 20, the thermal energy absorbed byhousing 16 tends to be retained thereby due to the insulating effect ofthe outer blanket. At the same time, the mirror (not shown) on the outersurface of leg 14 radiates the thermal energy present on housing 16 outto space, reduces and stabilizes the temperature of housing 16. Thus,improved temperature stability is provided for housing 16. For example,housing 16 is maintained at a temperature of about 240 K using thislatter configuration just described.

A cooler in accordance with the present invention was built and testedto determine its relative cooling capacity in comparison with thetheoretical design considerations noted above. The exterior surfaces ofhousing 16 were completely encased in an insulation blanket similar toblanket 25. Any heat absorbed by the insulation blanket entirelyencasing housing 16 when exposed to the sun is absorbed by housing 16and is prevented from being radiated out to space by this sameinsulation blanket. Thus, to cool housing 16 the housing is hard-mountedto the spacecarft, that is, by thermally conductive mounting means suchas bolts or the like. ln this instance, the cooler housing 16 ismaintained substantially at the same temperature as the spacecraft. Thistemperature has been calculated to be approximately 300K.

A bell jar, serving as a test chamber for the cooler, was provided withan inner liquid nitrogen shroud and a liquid helium shroud in separatetest configurations to simulate the thermal sink of space for each oftwo environmental simulations. The liquid nitrogen shroud provided athermal sink temperature of approximately 80K, while the liquid heliumshroud provided a thermal radiation sink temperature of approximately 35K. These shrouds have an approximate thermal radiation absorptivityfactor of 0.9 as compared to a thermal radiation absorptivity of 1.0 fordark space. These differences between the test environment and theactual space environment can be compensated for mathematically to showthat the tested results are essentially equivalent to a cooler in aspace environment.

The results of tests performed on the ground-based cooler conform veryclosely to the calculated predictions.

The thermal inputs to the test cooler at an altitude of 600 nauticalmiles, are such that housing 16 is expected to have a temperature of300K while the second stage is expected to have a thermal input of 374milliwatts due to the energy absorbed by the second surface mirror. Thethermal input to the second surface mirror comes from the earths selfemitted energy and earth reflected solar energy. The thermal inputs tothe third stage are approximately 5 milliwatts due to detector heatdissipation with a radiation detector mounted in thermal contact withthe third stage. These thermal inputs were applied directly to thetested cooler. Thus, to maintain the housing at 300K, heater wires wereattached to the outer surface of housing 16 adjacent leg 13, and toprovide the 374 milliwatt input, heater wires were attached to thesecond stage at leg 13 adjacent the inner surface 27 thereof. Fivemilliwatts of thermal energy were applied to the third stage by a smallresistor attached thereto. The second stage heater was maintained at aconstant temperature while the third stage heater was maintained at aconstant heat dissipation level.

With a shroud temperature of 80K, the second stage was maintained at163K and the detector dissipation thermal input was maintained atapproximately 5 milliwatts. With these inputs the third stage measured99K.

Extrapolating this measured temperature with respect to a space sink at4K, the third stage has a temperature of 80K. A second test with ashroud temperature at 81K and a second stage temperature at 163K and nodetector dissipation thermal input on the third stage, the third stagehad a measured temperature of 94K. By extrapolation the third stagetemperature was computed to be K relative to a space sink of 4K. Withthe liquid helium shroud temperature of 35K and a second stagetemperature of 163K and no detector dissipation themial input on thethird stage, the third stage had a measured temperature of K. This thirdstage temperature was computed to be 71.5K relative to a space sink of4K. Using a 5 milliwatt thermal input on the third stage this computedtemperature is raised to 8l.5l(.

According to these simulated tests, it has been shown that a cooleraccording to this invention, can be provided which has a third stagetemperature as low as approximately 70K. It will be appreciated by thoseskilled in this art that these tests are good simulations of operatingconditions of a cooler in a space orbit.

It should be noted that the entire cooler assembly size and testconfiguration is small, requiring an envelope volume of eight inches byseven inches by seven inches. The approximate cooler assembly weight is1 pound. Further, the size of the radiator surface area 27 of leg 13 ofthe cooler constructed in accordance with the preferred embodiment ofthe present invention is 31.5

square inches. The inside area of leg 14 is 39.9 square inches, and thearea of member 12 is 9.0 square inches.

In order to provide an even more precise temperature control of thethird stage member 12, a heater may be provided in thermal contacttherewith to raise the temperature of member 12 to some predeterminedtemperature above the normal operating temperature of member 12. In thismanner, the temperature of member 12 can be maintained within a half ofa degree. Thus, with the cooler member 12 having a normal operatingtemperature in the range of 7080K, a detector normally operating at atemperature of 95K could be employed therewith.

It will be understood that the size of a space cooler made according tothis invention will depend on the altitude at which it will orbit aswell as the orientation it assumes, particularly with respect to the sunand earth. The preferred embodiment described above illustrates the sizeand particular dimensions of a cooler at an altitude of 600 miles.

It will now be appreciated that in accordance with the present inventiona cooler can be provided which provides very efficient coolingoperation. According to the principles of the present invention, acombination of staged radiation shielding and surface finishes havingpreselected spectral properties to certain wavelengths ranges provide anextremely efficient cooler having a relatively large thermal coupling tospace whereby cooler temperatures previously unattainable are achieved.

What is claimed is:

l. A passive device for maintaining an article at a preselectedtemperature in the presence of a thermal radiation sink, comprising:

an article cooling and supporting member, said member including asurface exposed to the ambient, said surface having high emissivity andlow reflectivity to radiation within a first wavelength range when saidmember is at said preselected temperature, said article and said memberbeing in conductive thermal relationship, and

shielding means adjacent to and in conductive and radiation thermalisolation from said member, said shielding means including a firstsurface having high absorptivity and low reflectivity of thermalradiation within said first wavelength range and low absorptivity andhigh reflectivity of thermal radiation within a second wavelength range,said shielding means substantially preventing the impingement of thermalradiation essentially within said first and second wavelength ranges onsaid member, said member being cooled by radiation to said thermal sinkand said shielding means being cooled by radiation and reflection ofthermal energy essentially within said first and second wavelengths,respectively, to said thermal sink.

2. The device of claim 1 wherein said shielding means includes a secondsurface having high reflectivity and low emissivity of thermal radiationwithin said first wavelength and high absorptivity and low reflectivityof thermal radiation within said second wavelength, said second surfacebeing shielded from impingement by thermal radiation essentially withinsaid second wavelength,

said member being disposed in the field of view of said second surfaceand beyond the field of view of the other surface of said shieldingmeans, said shielding means surfaces having a temperature at whichthermal radiation is emitted essentially within said first wavelengthrange.

3. The device of claim 2 wherein said second surface is specular.

4. The device of claim 1 wherein said one surface is a second surfacemirror in thermal conductive relationship with said shielding means.

5. The device of claim 1 wherein said shielding means includes a firstradiation shield nested within a second radiation shield in thermalisolation therefrom, said one surface being disposed on said firstshield.

6. A passive device for maintaining an article at a preselectedtemperature in the presence of a thermal sink, comprising:

an article cooling and supporting member including a surface exposed tothe ambient, said surface having high emissivity and low reflectivity toradiation within a first wavelength'range when said member is at saidpreselected temperature, said article and said member being in thermalconductive relationship, first radiation shield surrounding saidsupporting member and secured thereto in thermal isolation, said firstshield including a first surface having high absorptivity and lowreflectivity of thermal radiation within said first wavelength range andhigh reflectivity and low absorptivity of thermal radiation within saidsecond wavelength range, and further including a second surface havinghigh reflectivity and low absorptivity of thermal radiation within saidfirst wavelength range, and low reflectivity and high absorption ofthermal radiation within said second wavelength range, said firstsurface being disposed beyond the field of view of said supportingmember surface, said second surface being disposed within the field ofview of said first surface and said supporting member surface, and

a second radiation shield surrounding and receiving said first radiationshield in nested relationship, said first and second shields beingsecured in thermal isolation with respect to each other, said secondshield having the outer surfaces thereof exposed to the ambient,

the temperature of the respective shields and supporting member beingprogressively lowered and cooled when said supporting member surfacefaces said thermal sink, said first surface is exposed to radiationwithin both said first and second wavelengths, and said second surfaceis exposed to radiation within essentially said first wavelength.

7. The device of claim 6 wherein said first and second radiation shieldsare L-shaped, and wherein said first surface is disposed on the innersurface of one leg, and said second surface is disposed on the innersurface of the other leg, said supporting member being disposed adjacentthe junction of the two legs.

8. The device of claim 6 wherein said first surface is a second surfacemirror secured in thermally conductive relationship to said one leg.

9. The device of claim 6 wherein said second surface is specular.

10. The device of claim 6 wherein said second radiation shield hassecured thereto on a portion of said outer surfaces a layer ofinsulation, a second portion of said outer surfaces having highreflectivity and low absorptivity of radiation within said firstwavelength range, and low reflectivity and high absorptivity ofradiation within said second wavelength range.

1. A passive device for maintaining an article at a preselectedtemperature in the presence of a thermal radiation sink, comprising: anarticle cooling and supporting member, said member including a surfaceexposed to the ambient, said surface having high emissivity and lowreflectivity to radiation within a first wavelength range when saidmember is at said preselected temperature, said article and said memberbeing in conductive thermal relationship, and shielding means adjacentto and in conductive and radiation thermal isolation from said member,said shielding means including a first surface having high absorptivityand low reflectivity of thermal radiation within said first wavelengthrange and low absorptivity and high reflectivity of thermal radiationwithin a second wavelength range, said shielding means substantiallypreventing the impingement of thermal radiation essentially within saidfirst and second wavelength ranges on said member, said member beingcooled by radiation to said thermal sink and said shielding means beingcooled by radiation and reflection of thermal energy essentially withinsaid first and second wavelengths, respectively, to said thermal sink.2. The device of claim 1 wherein said shielding means includes a secondsurface having high reflectivity and low emissivity of thermal radiationwithin said first wavelength and high absorptivity and low reflectivityof thermal radiation within said second wavelength, said second surfacebeing shielded from impingement by thermal radiation essentially withinsaid second wavelength, said member being disposed in the field of viewof said second surface and beyond the field of view of the other surfaceof said shielding means, said shielding means surfaces having atemperature at which thermal radiation is emitted essentially withinsaid first wavelength range.
 3. The device of claim 2 wherein saidsecond surface is specular.
 4. The device of claim 1 wherein said onesurface is a second surface mirror in thermal conductive relationshipwith said shielding means.
 5. The device of claim 1 wherein saidshielding means includes a first radiation shield nested within a secondradiation shield in thermal isolation therefrom, said one surface beingdisposed on said first shield.
 6. A passive device for maintaining anarticle at a preselected temperature in the presence of a thermal sink,comprising: an article cooling and supporting member including a surfaceexposed to the ambient, said surface having high emissivity and lowreflectivity to radiation within a first wavelength range when saidmember is at said preselected temperature, said article and said memberbeing in thermal conductive relationship, a first radiation shieldsurrounding said supporting member and secured thereto in thermalisolation, said first shield including a first surface having highabsorptivity and low reflectivity of thermal radiation within said firstwavelength range and high reflectivity and low absorptivity of thermalradiation within said second wavelength range, and further including asecond surface having high reflectivity and low absorptivity of thermalradiation within said first wavelength range, and low reflectivity andhigh absorption of thermal radiation within said second wavelengthrange, said first surface being disposed beyond the field of view ofsaid supporting member surface, said second surface being disposedwithin the field of view of said first surface and said supportingmember surface, and a second radiation shield surrounding and receivingsaid first radiation shield in nested relationship, said first andsecond shields being secured in thermal isolation with respect to eachother, said second shield having the outer surfaces thereof exposed tothe ambient, the temperature of the respective shields and supportingmember being progressively lowered and cooled when said supportingmember surface faces said thermal sink, said first surface is exposed toradiation within both said first and second wavelengths, and said secondsurface is exposed to radiation within essentially said firstwavelength.
 7. The device of claim 6 wherein said first and secondradiation shields are L-shaped, and wherein said first surface isdisposed on the inner surface of one leg, and said second surface isdisposed on the inner surface of the other leg, said supporting memberbeing disposed adjacent the junction of the two legs.
 8. The device ofclaim 6 wherein said first surface is a second surface mirror secured inthermally conductive relationship to said one leg.
 9. The device ofclaim 6 wherein said second surface is specular.
 10. The device of claim6 wherein said second radiation shield has secured thereto on a portionof said outer surfaces a layer of insulation, a second portion of saidouter surfaces having high reflectivity and low absorptivity ofradiation within said first wavelength range, and low reflectivity andhigh absorptivity of radiation within said second wavelength range.